Process for peroxide bleaching of pulp using MgO particles

ABSTRACT

A process for peroxide bleaching of pulp using magnesium oxide as sole alkaline source wherein said pulp is bleached in the presence of hydrogen peroxide for a maximum period of 180 minutes and achievement of a maximum target ISO brightness of 65 in regard to freshly prepared pulp characterized in that said magnesium oxide is utilized as MgO particles having a particle size of 5-500 microns and a particle surface area (PSA) of between 20-60 m 2  /g. By using such parameters, a peroxide bleaching process may be carried out most efficiently on a commercial scale.

FIELD OF THE INVENTION

THIS INVENTION relates to a process for peroxide bleaching of pulp.Pulps which may be bleached in the process of the invention includelignocellulose pulp which may be produced mechanically andchemi-mechanically with yields in the region of greater than 75% whichare otherwise known as high yield pulps.

BACKGROUND OF THE INVENTION

In a conventional peroxide bleaching process, sodium hydroxide is usedas an alkali source. To achieve a desired brightness with maximumefficiency, auxiliary substances are also used. Such auxiliarysubstances include sodium silicate, magnesium sulphate and chelatingagents inclusive of DTPA (sodium salt of diethylene triamino pentaaceticacid).

Reference may be made to a prior art article by Soteland et al., 1988,TAPPI Proceedings 231-236, which describes a peroxide bleaching processwhich utilises magnesium oxide as a sole alkaline source. The pulp waspretreated with DTPA and magnesium oxide particles were utilised in asize range of 1.00 mm-0.25 mm or smaller. The magnesium oxide were alsoused in a concentration of 2-3% based on the dry weight of the pulp. TheMgO used in the process was light-burnt MgO and finely crushed. It wasfound that brightness levels obtained were very close to that which wasachieved by conventional bleaching using NaOH. In the bleaching process,the pulp was diluted to form a pulp suspension and the amount of MgO wasadded to the suspension under vigorous stirring. Hydrogen peroxide wassubsequently added to the suspension at a concentration of 3% based onthe weight of the pulp. This reference also made the observation thatcoarse particles are less effective as an alkaline source duringperoxide bleaching.

Having regard to the abovementioned reference, an observation was alsomade in the corresponding patent specification DE3617942 that use of MgOas sole alkaline source considerably simplified the bleaching processsince sodium hydroxide as alkaline source and auxiliary chemicals suchas sodium silicate could be omitted.

Another advantage of using MgO as sole alkaline source was that only asmall amount of waste is produced in the bleaching plant. Thus, forexample, in integrated mills which produce magnesium sulphite pulp andperoxide bleached high-yield pulps, the used bleaching liquor iscombusted and the MgO may be recovered for re-use.

However, the use of MgO as sole alkaline source in a peroxide bleachingprocess has not as yet achieved widespread commercial acceptance becausealthough the principle of utilising MgO as sole alkaline source wasdescribed in the Soteland at al. references, the means of reducing theprinciple to practice on a commercial scale has not yet been fullyelucidated.

SUMMARY OF THE INVENTION

Surprisingly, it has now been discovered that commercial usage ofmagnesium oxide as a sole alkaline source in peroxide bleaching of woodpulp may be achieved by employing MgO particles with a particle size ofless than 500 micron and more preferably less than 75 micron and havingparticle surface area (PSA) of between 20-60 m² /g and more preferablybetween 30-50 m² /g. By using such parameters, an efficient peroxidebleaching process may be carried out most efficiently on a commercialscale which may be achieved within a maximum bleaching time of 180minutes and achievement of a maximum target of ISO brightness of 65 inregard to freshly prepared pulp.

Utilizing MgO with parameters outside those stated above will result ina less efficient bleaching process leading to higher usage of chemicalsand therefore higher operating costs.

The dosages of MgO that may be utilised in the process of the inventionis 0.3-2% based on the weight of the pulp.

The amount of hydrogen peroxide that may be utilised in the process ofthe invention is from 1-5% based on the weight of the pulp.

To achieve maximum efficiency, the MgO particles are preferably added tothe pulp in the form of a powder or slurry prepared in situ.

Preferably the MgO is added to the pulp simultaneously with the peroxideor prior to the addition of the peroxide.

Chelating agents also may be used in the process of the invention andsuch chelating agents may comprise DTPA, EDTA or HEDTA (hydroxy-ethylenediamine tetracetic acid). Preferably the chelating agent is added to thepulp simultaneously with addition of MgO particles, as well as prior toaddition of MgO particles.

Bleaching times of 60-180 minutes may also be utilised by the process ofthe invention to achieve a target ISO brightness of 55-65.

BRIEF DESCRIPTION OF DRAWINGS

In several preferred embodiments concerning the process of the inventionwhich are discussed hereinafter in relation to Experiments 1 and 2;

FIG. 1 is a graph showin the effect of particle size on CCS (ColdCaustic Soda) pulp and more specifically showing particle size vsbrightness at different times;

FIG. 2 refers to the results of Experiment 2 whereby various samples areplotted against final brightness;

FIG. 3 also refers to the result of Experiment 2 and shows the effect ofsurface area on CCS pulp and more specifically showing particle size vsbrightness at different times; and

FIG. 4 shows the results of FIG. 3 when plotted against time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXPERIMENT 1 EFFECT OF MGOPARTICLE SIZE ON THE BLEACABILITY OF CCS PULP

Introduction

This work was performed to establish a relationship between MgO particlesize and alkali performance in the bleaching of CCS (Cold Caustic Soda)wood pulp. Four grades of MgO were trialled, each one identified by itsparticle size distribution. Each sample of MgO had approximately thesame surface area. Particle size and surface area for each of thesamples is given in Table 1.

Summary of bleaching work

CCS (chemi-mechanical pulp), pretreated with DTPA to remove metal ions,was retrieved from the washers in the bleach plant at the Boyer mill. Anequivalent mass of 20 grams OD of pulp was weighed out and placed in aplastic breaker. DTPA was then added as 0.15% v/w on the pulp and mixed.MgO as 0.4% wow, enough water to give a stock consistency of 12% andperoxide as 1.6% v/w on pulp was added and mixed for 2 minutes. The pulpwas wrapped in plastic bags and placed into a constant temperature waterbath at 65° C. A 3 gram OD sample was removed from the bath at intervalsof 2, 3 and 4 hours. This was then made into a brightness hand sheetusing the standard Boyer pulp mill method. These were dried overnight ina constant temperature/humidity room and tested for ISO brightness. Thisprocedure was repeated for all MgO samples as well as with control pulpcontaining no MgO (sample J)

Results

The results of this study indicate that particle size is a key parameterfor achieving efficient peroxide bleaching of chemic-mechanical pulp.The results shown in Table 2 and FIG. 1 of this study indicate that anMgO particle size of <75μ (samples G and F, d90=65 and 35 respectively)is required to achieve a target brightness for a given retention time of2, 3 or 4 hours.

To achieve an equivalent brightness with samples C (d90=1500) or D(d90=3500), the chemical dosages of MgO and H₂ O₂ would need to beincreased.

EXPERIMENT 2 EFFECT OF MGO PARTICLE SURFACE AREA ON THE BLEACHABILITY OFCCS PULP

Introduction

This work was performed to establish a relationship between MgO particlesurface area and alkali performance in the bleaching of CCS (ColdCaustic Soda) wood pulp. Five grades of MgO were trialled, each oneidentifiable by its particle surface area. Each sample of MgO hadapproximately the same particle size. Particle size and surface areadata for each of the samples is given in Table 3.

Summary of bleaching work

CCS (chemi-mechanical) pulp, pre-treated with DTPA to remove metal ions,was retrieved from the washers in the bleach plant at the Boyer mill.For each sample, a mass of 10 g O.D. pulp was placed into a beaker andthe approximate mass of chemicals added. The pulp was mixed for 2minutes in a bench top mixer. The pulp was then wrapped in plastic bagsand placed into a constant temperature water bath at 65° C. After twohours retention, the samples were removed from the bath and divided intotwo. Half the sample was returned to the bath for a further hour ofreaction while the other half was made into 5 gram brightness handsheets. These were dried overnight and then tested for ISO brightness.The work was repeated with samples taken at 2, 3 and 4 hours.

Results

In the previous study (Experiment 1), we determined that MgO particlesize was important for peroxide bleaching efficiency. The results ofthis study indicate that particle surface area is also a key parameterfor achieving maximum brightness for a given chemical dose. The resultsfrom these two independent studies (Tables 4 and 5, FIGS. 2 and 3)indicate that a surface area in the range 30-50 m² /g (samples B and C)is required to achieve maximum brightness for a given retention time andchemical dose. Surprisingly, when the surface area is either decreasedor increased, the peroxide bleaching efficiency is reduced as indicatedin FIGS. 2 and 3 by the bell shaped curves with brightness plateausbetween samples B and C. To achieve an equivalent brightness to samplesB and C with samples A, D or E, the chemical charges of H₂ O and MgOwould need to be increased.

The results in FIG. 3, when plotted against time (FIG. 4), appear toindicate that a similar brightness will be achieved with four of thefive samples when the bleaching time is extended indefinitely. However,indefinite bleaching time is not a commercial reality and there is aclear benefit, based on these results, in employing MgO particles with aspecific size and surface area. In fact, if MgO particles, withparameters outside those stated in this document are used, then thetarget brightness may not be achieved without increasing chemical doserates.

                  TABLE 1                                                         ______________________________________                                                    Particle size d90                                                 Sample      micron      Surface area m.sup.2 /g                               ______________________________________                                        F            35         38                                                    G            65         35                                                    H           1500        30                                                    I           3500        30                                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                Surface Area                                                                          Brightness                                                    Sample    m.sup.2 /g                                                                              2 hr       3 hr 4 hr                                      ______________________________________                                        F         <40       61.39      62.17                                                                              62.88                                     G         <75       61.22      61.94                                                                              62.69                                     H         <2000     56.85      57.98                                                                              59.17                                     I         <5000     56.32      56.19                                                                              56.97                                     J         0         54.3       55.03                                                                              55.04                                     ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                    Particle size d90                                                 Sample      micron      Surface area m.sup.2 /g                               ______________________________________                                        A           14           1                                                    B           10          35                                                    C           10          43                                                    D           15 (d90 = 70)                                                                             96                                                    E           11          142                                                   ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                               Surface Area  Brightness                                               Sample   m.sup.2 /g      2 hr   3 hr                                          ______________________________________                                        A         1              58.5   59.1                                          B        35              60.2   60.5                                          C        43              60.1   60.6                                          D        98              58.3   59.0                                          E        142             56.8   58.8                                          ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                                Surface Area                                                                          Brightness                                                    Sample    m.sup.2 /g                                                                              2 hr       3 hr 4 hr                                      ______________________________________                                        A          1        56.02      57.75                                                                              58.54                                     B         35        58.89      60.58                                                                              60.96                                     C         43        59.17      31.37                                                                              61.09                                     D         98        58.15      59.1 60.29                                     E         142       57.45      59.13                                                                              60.27                                     ______________________________________                                    

LEGENDS

                  TABLE 2                                                         ______________________________________                                        Bleaching conditions                                                          ______________________________________                                               MgO% w/w on oven dry pulp = 0.4%                                              H.sub.2 O.sub.2 - 1.6%                                                        DTPA = 0.15%                                                                  Temperature = 65° C.                                                   Initial brightness 47.1                                                ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Bleaching conditions                                                          ______________________________________                                               MgO% w/w on oven dry pulp = 0.3%                                              H.sub.2 O.sub.2 = 1.8%                                                        DTPA = 0.1%                                                                   Temperature = 65° C.                                                   Initial brightness 43.5                                                ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        Bleaching conditions                                                          ______________________________________                                               MgO% w/w on oven dry pulp = 0.4%                                              H.sub.2 O.sub.2 = 1.6%                                                        DTPA = 0.15%                                                                  Temperature - 65° C.                                                   Initial brightness 47.1                                                ______________________________________                                         FIG. 1                                                                        Effect of particle size on CCS pulp  Particle size vs brightness at           different times.                                                              FIG. 2                                                                        Sample number vs final final brightness.                                      FIG. 3                                                                        Effect of surface area on CCS pulp  Surface area vs brightness at             different                                                                     FIG. 4                                                                        Effect of surface area on CCS pulp  Time vs brightness for different          surface areas                                                            

What is claimed is:
 1. A process for peroxide bleaching of pulpcomprising the steps of:adding magnesium oxide and hydrogen peroxide tosaid pulp, said magnesium oxide is utilized as MgO particles having aparticle size of 5-500 microns and a particle surface area (PSA) ofbetween 20-60 m² /g; and bleaching said pulp for a period of 60-180minutes to achieve a pulp ISO brightness of 55-65.
 2. The process asclaimed in claim 1 wherein said MgO particles have a particle size ofless than 75 microns.
 3. The process as claimed in claim 1 wherein theMgO particles have a particle size surface area of between 30-50 m² /g.4. The process as claimed in claim 1 which utilizes a dosage of MgOwhich is 0.3-2% based on the weight of the pulp.
 5. The process asclaimed in claim 1 wherein the amount of hydrogen peroxide utilizedcomprises 1-5% based on the weight of the pulp.
 6. The process asclaimed in claim 1 wherein the MgO particles are added to the pulp inthe form of powder or slurry prepared in situ.
 7. The process as claimedin claim 1 wherein the MgO particles are added to the pulpsimultaneously with the addition of peroxide.
 8. The process as claimedin claim 1 wherein the MgO particles is added to the pulp prior to theaddition of peroxide.
 9. process as claimed in claim 1 furthercomprising the step of adding:a chelating agent to the pulp selectedfrom DTPA, EDTA or HEDTA before said bleaching step.
 10. The process asclaimed in claim 9 wherein the chelating agent is added to the pulpsimultaneously with the addition of MgO particles.
 11. The process asclaimed in claim 9 wherein the chelating agent is added to the pulpprior to the addition of MgO particles.